July 9, 1999 The high-speed portion of the solar wind achieves its unexpectedly high velocity -- up to 500 miles per second -- by "surfing" magnetic waves in the Sun's outer atmosphere, according to observations made by two spacecraft during John Glenn's return to space.
For 37 years, solar scientists have been puzzled by the fact that the high-speed solar wind travels twice as fast as predicted by theory. Observations and theoretical analyses have discovered a surprising explanation for this mystery: magnetic waves. The observations were made using instruments aboard NASA's Spartan 201 spacecraft, deployed from the Space Shuttle during the STS-95 mission, and the international Solar and Heliospheric Observatory (SOHO).
"The mystery was first presented by the Mariner 2 spacecraft in 1962, the same year as Glenn's first flight," said Dr. Marcia Neugebauer of NASA's Jet Propulsion Laboratory, Pasadena, CA, the co-principal investigator of the solar wind instrument on Mariner 2. "The new observations made by SOHO and by the Spartan 201 mission during Glenn's return to space put us much closer to finally unraveling the mystery of the acceleration of the solar wind."
The outermost solar atmosphere, or corona, is an extremely tenuous, electrically charged gas that is seen from Earth only during a total eclipse of the Sun by the moon, when it appears as a shimmering white veil surrounding the black lunar disk. Using Spartan and SOHO, scientists have detected rapidly vibrating magnetic fields within the corona that form magnetic waves that appear to accelerate the solar wind.
"These vibrating magnetic waves give solar wind particles a push, just like an ocean wave gives a surfer a ride," said Dr. John Kohl, a senior astrophysicist at the Smithsonian Astrophysical Observatory in Cambridge, MA, and the principal investigator for ultraviolet spectrometers aboard SOHO and the Spartan 201.
The electrical charges of solar wind particles, or ions, force them to spiral around invisible magnetic lines in the corona as they rush into space. When the lines vibrate, as they do in a magnetic wave, the spiraling ions are accelerated out and away from the Sun. The presence of magnetic waves in the corona was inferred by observing the motions of these solar wind ions. "We were quite surprised to find that heavier oxygen ions actually moved faster than lighter hydrogen ions," said Dr. Steven Cranmer of the Harvard-Smithsonian Center for Astrophysics in Cambridge, MA. "The best explanation for this is a magnetic field line that wiggles back and forth in the same amount of time that oxygen ions take to spiral around it. Just as a child riding on a swing moves higher if someone pushes with the right rhythm, the resonant magnetic waves give a boost to the oxygen ions." The scientists believe there are magnetic waves in the corona with many different "wiggling periods," or frequencies. These waves accelerate various solar wind particles that spiral around the field lines at different rates. The observations also will help scientists better understand solar regions called coronal holes. "Solar winds from these regions enhance energetic electrons in the Earth's Van Allen radiation belts, which can sometimes affect the electrical systems aboard Earth-orbiting satellites," said Joseph W. Hirman, Chief of the Division for Space Weather Operations at the Space Environment Center operated by the National Oceanic and Atmospheric Administration in Boulder, CO. Even with this major discovery, there are questions left to answer. "The observations have made it abundantly clear that heavy particles like oxygen 'surf' on the waves, and there is also mounting evidence that waves are responsible for accelerating the hydrogen ions, the most common constituent of the solar wind," Cranmer said. "Other common particles, such as helium, have never been observed in the accelerating part of the corona, and new observations also are needed to refine our understanding of how the waves interact with the solar wind as a whole."
The SOHO mission is sponsored by NASA and the European Space Agency. This research was published in the June 20 edition of the Astrophysical Journal.
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